The halftone phase shift mask generally is considered an improvement of the chrome phase shift mask. The halftone phase shift mask is called an attenuated phase shift mask, or a "t.pi." phase shift mask (t means transmittance).
The structure and principles of the phase shift mask will be explained hereinafter, referring to FIGS. 1A to 1D.
First, as illustrated in FIG. 1A, a halftone phase shift mask is formed by depositing halftone pattern material layer 2 of, such as, chrome oxide CrO, on transparent substrate 1 of, such as, quartz or glass. Halftone pattern material layer 2 has properties such as to shift the phase 180.degree. and transmit only 4 to 30% of the light incident thereto.
When the halftone shift mask positioned over wafer 4 having masking material 3 such as photoresist coated on the surface thereof is irradiated with light from above, the surface of masking material 3 exhibits an intensity profile of light as illustrated in FIG. 1B. That is, an open region without halftone pattern 2 thereon transmits light through transparent substrate 1 to exhibit a positive intensity profile of the light on masking material 3. On the other hand, the portion having halftone pattern 2 deposited thereon transmits 4 to 30% of the light with the phase shifted 180.degree. to exhibit a negative value.
As illustrated in FIG. 1B, the intensity of light on masking material 3 has a rectangular profile. As illustrated in FIG. 1C, however, the intensity of light on wafer 4 has not a rectangular profile, but a sinusoidal profile. That is, beside the "overshoot" of the main lobe, side lobes are present, which repeat a pattern of "overshoot" and "undershoot."
In FIG. 1C, I.sub.1 indicates an intensity profile of light formed by light passed through the open region with no halftone pattern material 2 deposited thereon, and I.sub.2 indicates an intensity profile of light formed by the light passing through halftone pattern 2.
Illustrated in FIG. 1D is a final intensity profile of light which is a composite of the intensity profiles of I.sub.1 and I.sub.2. According to FIG. 1D, the intensity of light in the side lobes are weak compared to the intensity of light at the main lobe that the side lobe intensity can be considered negligible.
Since a conventional, common chrome phase shift mask only has the chrome pattern layer without halftone pattern material layer 2, the intensity of light on wafer 4 also is the same as I.sub.1 of FIG. 1C. The intensity profiles of light of FIGS. 1C and 1D illustrate a case in which the light has been focused more accurately. In most cases, however, there remains a high probability of the light being defocused.
An advantage of the halftone phase shift mask over the conventional chrome phase shift mask is that the halftone phase shift mask is less likely to be defocused. The reason for this advantage is to be explained hereinafter.
FIG. 2A is a graph illustrating intensity profiles of light for cases of being best focused and defocused using a halftone phase shift mask. FIG. 2B is a graph illustrating intensity profiles of light for cases of being best focused and defocused using a conventional chrome phase shift mask.
According to FIG. 2A, it can be seen that each of the main lobes has a higher intensity of light than respective side lobes, irrespective of the degree it has been defocused. According to FIG. 2B, however, it can be seen that the side lobes have a higher intensity of light than respective main lobes in the case of being seriously defocused. In such cases, patterns entirely different from what has been intended can form on masking material 3 of FIG. 1A.
As has been explained, the conventional halftone phase shift mask has an advantage in that it cannot only eliminate the side lobes that form unwanted patterns, but also carry out a more accurate patterning compared to other conventional phase shift masks even under a condition of being defocused, by composing the intensity of light on the open region with the intensity of light on halftone pattern material layer 2 having a phase of 180.degree. and 3 to 40% transmissivity.
FIGS. 1A-1D and FIGS. 2A and 2B, however, illustrate cases when formation of only one pattern is taken into account, but not cases when formation of many patterns is taken into account. FIG. 3A is a plan view of a halftone phase shift mask with six open regions therein for forming six patterns on wafer 4 of FIG. 1A. FIG. 3B is a section across line 3B--3B of FIG. 3A, and FIG. 3C illustrates overlap between side lobes of the light passed through halftone pattern material layer 2 having two open regions therein.
As illustrated in FIG. 3B, of the intensity profiles of the light passed through each of the open regions of the halftone phase shift mask, the intensity profiles of the light of the side lobes can overlap. When the overlap is extensive, new side lobes having almost the same intensity of light with each of the desired main lobes can be formed as illustrated in FIG. 3D. As a result, even though patterns illustrated in FIG. 3E were desired to have been formed on masking material 3, unwanted anomalous patterns 5 as illustrated in FIG. 3F can be formed on masking material 3. Accordingly, unwanted final patterns can be formed on wafer 4.
As has been explained, unwanted anomalous patterns 5 on masking material 3 can, or cannot, be formed depending on distances between the open regions of the halftone phase shift mask. That is, if the open regions are spaced apart with sufficient distances, overlap between the side lobes will not occur. With the recent trend of large scale integration of all semiconductor elements, however, as many patterns as possible should be arranged within a given area. Accordingly, as spaces between patterns become narrower, greater overlap between side lobes may occur as illustrated in FIGS. 3A to 3F, which further causes formation of unwanted anomalous patterns 5.
FIG. 4 illustrates the degree of overlap of the side lobes formed depending on the variation of a distance X between two adjacent open regions in a halftone phase shift mask. According to FIG. 4 it can be seen that, when distance x between the open regions is varied from 0.36 .mu.m to 1.0 .mu.m, the degree overlap of the two adjacent side lobes becomes greatest when distance X is 0.84 .mu.m.
As has been explained, the conventional halftone phase shift mask has a problem in that it can cause undesired anomalous patterns on the masking material as well as on the wafer because overlap of the side lobes of light passed through halftone pattern material layer 2 form new side lobes having a high intensity of light.